Mucosal surfaces such as the intestinal epithelium provide a complex challenge to the immune system, as they must maintain effective immunity against potential pathogen attack while also tolerating commensal microorganisms and dietary antigens. The balance between tolerance and immunity is mediated in large part by the specialized T cell populations, regulatory T cells (Tregs) and Th17 cells. Disruption of the balance between these subsets has been heavily implicated in causing colitis through studies in both mice and humans. Furthermore, Th17 cells are emerging as contributors to many other inflammatory and autoimmune disorders. Understanding how Th17 responses are initiated and regulated is therefore critical to our understanding of both intestinal immunity and chronic inflammatory disease. Our long term goal is to understand how Dendritic Cells (DCs) and other antigen presenting cells initiate and maintain the correct balance of T-helper cell subsets. The objective in this application is to establish how DCs regulate T cell responses to TGF- and control Th17 cell differentiation. The rationale for this work is that understanding this process has the potential to translate into new therapies for Th17-mediated diseases. Our central hypothesis is that DC expression of ?v8 and subsequent activation of TGF- controls induction and maintenance of Th17 responses. Based on published work and preliminary data, this hypothesis will be tested in three specific aims: (1) Determine how ?v integrins expressed by DCs regulate Th17 cell differentiation, particularly the generation of IL17/ IFN-?-expressing cells associated with autoimmunity. Conditional knockout mice will be used to study the role for ?v integrins in vivo, and results will be extended to humans though primary cell culture. (2) Understand how 8 expression is regulated in DCs during homeostatic regulation and following infection. (3) Determine the mechanisms by which ?v8 function is regulated on DCs using cell biology and biochemical approaches. This approach is innovative as it focuses on the role of DCs in 'licensing' T cell responses to TGF-. The proposed work is significant because it will provide a much needed understanding of how DCs promote the initiation of Th17 cells in the intestine and regulate their subsequent differentiation in response to environmental and infectious challenges.